Project Summary Brucella spp. are bacteria that naturally infect a variety of domesticated and wild animals leading to abortions and sterility, and these bacteria are also capable of causing debilitating human infections, which often result from human exposure to infected animals and animal products. Brucella spp. are considered threats as potential biological weapons. Importantly, antibiotic treatment against brucellosis is prone to disease relapse, and there is currently no safe and effective vaccine to protect humans against infection with Brucella. The brucellae are intracellular pathogens that reside within immune cells called macrophages where they replicate in a specialized compartment, and the capacity of Brucella to survive and replicate within macrophages is essential to their ability to cause disease. Over the last few years, our laboratory has characterized genetic pathways that are critical for the intracellular survival and pathogenesis of Brucella strains, and specifically, we have identified small regulatory RNAs (sRNAs) that are essential for Brucella virulence. Preliminary experiments have demonstrated that a family of sRNAs, called the AbcRs is required for the ability of B. abortus to chronically infect mice. When these genes encoding these two sRNAs (i.e., AbcR1 and AbcR2) are deleted, the resulting strain is highly attenuated, and moreover, we have determined that this deletion strain produces extremely high levels of Brucella immunogenic proteins. We hypothesize that the abcR1 abcR2 deletion strain can serve as a highly effective live, attenuation vaccine against B. abortus challenge, and the pilot studies outlined in this application will test this hypothesis. In the end, the information gleaned from these studies may be used to develop an effective vaccine against human Brucella infection.